By definition, a PN junction is the interface between two regions in a semiconductor crystal which have been treated (i.e. doped) so that one region is a P-type semiconductor and the other is an N-type semiconductor; it contains a permanent dipole charge layer (McGraw-Hill Dictionary of Scientific and Technical Terms: Sixth Edition 2003). More particularly, from a technical perspective, the P-type region includes “holes” arid the N-type region includes “electrons”. In this combination, the permanent dipole charge layer (i.e. a space charge layer) is located between the regions.
As its nomenclature suggests, the space charge layer between the P-type region and the N-type region will be charged. Further, it will have a depletion width, Wd, that is initially determined by the electrical characteristics of the P-type and N-type regions. Importantly, it is known according to the plasma dispersion effect that the index of refraction of a semiconductor material will change as its free carrier concentration is changed. Therefore, the effective refractive index, n, of the PN diode will change as the depletion width Wd is changed. It happens that these changes can be induced electronically by the application of an external voltage.
It is well known that semiconductor materials exhibit a phenomenon that is known as the plasma dispersion effect. In brief, this effect is related to the density of free electron carriers in a semiconductor material. More specifically, this free electron density is determined by the concentration of “electrons” in the N-type region of a PN junction, and by the concentration of “holes” in the N-type region of the PN junction. Of particular interest for the present invention is how the plasma dispersion effect changes the index of refraction of a semiconductor material, and the affect this change will have on an optical signal as it passes through a PN junction.
Along with a consideration of PN junctions as mentioned above, the characteristics of optical waveguides are also important for the present invention. In particular, the interest here is on the nature of light beams and their interaction with an optical waveguide. First, consider a single mode light beam which has no higher order modes and exhibits only what is generally referred to as the fundamental mode. As a distinguishing feature, it is well known that unlike a multi-mode light beam which always includes a fundamental mode together with higher order modes, a single mode light beam will follow a straight line path through an optical waveguide. On the other hand, a higher order mode light beam (e.g. second order mode) primarily will follow a sinusoidal path which passes back and forth across a center line through the optical waveguide due to mode propagation interference.
The present invention has recognized several possibilities from the technical considerations mentioned above that lead toward the use of an optical waveguide as a reverse bias switching/modulating diode. For one, the present invention recognizes that an optical waveguide, which is made of a semiconductor material (e.g. silicon), can be “doped” to create a PN junction. Specifically, both a P-type region and an N-type region, with a space charge layer therebetween, can be manufactured as an optical waveguide to effectively create a waveguide/diode. For another, the present invention recognizes that by introducing a higher order mode optical signal (e.g. second order) into the waveguide/diode, the sinusoidal beam path of the optical signal will cause it to transit back and forth through the space charge region. By changing the external voltage, the depletion width Wd and its corresponding effective index n of the diode will change, and the beam path of the optical signal will be cumulatively changed as it passes back and forth through the space charge region in the waveguide/diodes Moreover, this change in beam path can then be effectively used to selectively direct (i.e. switch) the optical signal as an output from the waveguide/diode onto either of two output optical waveguides.
In light of the above, it is an object of the present invention to provide a reverse bias switching/modulating diode wherein the switching element is itself an optical waveguide. Another object of the present invention is to provide a reverse bias switching/modulating diode that effectively provides for optical switching of higher order mode optical signals. Another object of the present invention is to control the loss/gain component ik of a modulator's index of refraction n+ik for the purpose of causing anisotropic changes in components of a light beam's flux energy (i.e. Poynting vector components P∥and P⊥), to thereby change the propagation distance of the light beam, λc, for eventually switching the light beam from one waveguide to another. Yet another object of the present invention is to provide a reverse bias switching/modulating diode that is easy to manufacture, is simple to use, and is comparatively cost effective.
In another aspect of the pertinent technology, the present invention gives consideration to both the phase characteristics and the losses of an optical signal as it transits through an optical waveguide/diode. Mathematically, these considerations are given in the expression for the waveguide/diode's index of refraction:n=(no+Δno)+i(αo+Δαo)
In this expression no and αo are measures of the intrinsic properties of the waveguide/diode. On the other hand, Δno and Δαo respectively account for a plasma dispersion effect in the PN junction (phase factor) and losses due to absorption effects in the waveguide/diode (loss factor). An important consequence here is that when these factors are considered together, the index of retraction n varies along a cross-section dimension of the waveguide/diode due to free electron and hole distributions variations around the PN junction.
As disclosed above, an optical signal with higher order modes will follow a sinusoidal path as it transits through an optical waveguide/diode. An important characteristic of this path is the propagation interference distance λc that is traveled by the optical signal, in a π cycle, as the optical signal travels from one side of the waveguide/diode to the opposite side of the waveguide/diode.
With the above in mind, when both phase and loss factors are considered, it is to be appreciated that the propagation interference distance λc can be expressed as:λc=λcn+Δλcn+Δλca where, λcn is a constant that is set by the physical characteristics of the waveguide/diode, while Δλcn and Δλca are consequences of the phase and loss factors presented in the expression given above for the waveguide/diode's index of refraction, n=(no+Δno)+i(αo+Δαo).
Specifically, in the expression for n given above, Δλcn is an increment that is controlled by Δno, and Δλca is an increment that is controlled by Δαo. Thus, during each transit of an optical signal through the PN junction of a waveguide/diode, the optical signal will experience a change in the propagation interference length Δλc due to both phase and loss factors: i.e. Δλc=Δλcn+Δλca.
Importantly, because the fundamental mode of an optical signal is less attenuated than the higher order modes in the depletion region, Wd, of the PN junction, the fundamental mode has relatively less propagation loss during a π cycle. Accordingly, with less propagation loss, the energy in the fundamental mode is more pronounced. Consequently, Δλcn remains relatively constant while Δλca increases with propagation distance and the difference between the two, Δλc, also increases. Stated differently, Δc is distance-dependent and is effectively stretched as it transits the waveguide/diode. Moreover, these changes (Δλca and Δλcn) are cumulative during the transit of the optical signal through the waveguide/diode.
From an energy perspective it is known that the propagation of a light beam's energy flux can be characterized by a Poynting vector, P. By definition, at any point in time, the Poynting vector will have both a magnitude and a direction. In the specific case when the light beam travels as a multimode optical wave on a zigzag path along the axis of a multimode optical waveguide, the Poynting vector can be resolved into a component P∥ that is directed along the axis of the multimode optical waveguide and a component P195  that is directed perpendicular to the axis.
When specifically considering the Poynting vector P of a light beam at the point where it is incident on a material having a spatial varying index of refraction n+ik along the waveguide cross-section, it is known that the loss/gain component ik of this index of refraction will introduce losses or gains that alter the direction of the vector P. In detail, it is known that under the influence of ik, the components and P∥ will P⊥ vary with different loss/gain values in an anisotropic manner. The important consequence here is that the direction of the Poynting vector can be controlled by altering the index of refraction n+ik.